Dry conversion of high purity ultrafine silicon powder to densified pellet form for silicon melting applications

ABSTRACT

A bulk silicon material for making silicon ingots, consisting of silicon pellets, and a method for making the pellets from an agglomerate-free source of high purity, ultra fine silicon powder by feeding a controlled amount of silicon powder into a pellet die, and dry compacting the powder at ambient temperature with pressure to produce a pellet that has a density of about 50-85% of the theoretical density of elemental silicon, a weight within a range of about 1.0 gram to about 3.0 grams, a diameter in the range of 10 mm to 20 mm and preferably of about 14 mm, and a height in the range of 5 mm to 15 mm and preferably of about 10 mm.

RELATED APPLICATIONS

This application is a continuation-in-part to pending application Ser.No. 10/413,774, filed Apr. 15, 2003, claiming benefit of U.S.Provisional Patent Application Ser. No. 60/372,980, filed Apr. 15, 2002,under 35 U.S.C. § 119(e).

FIELD OF INVENTION

This invention relates to raw materials used for melting to make siliconingots, in particular to a bulk silicon material consisting of siliconpellets make by compacting silicon powder without additives or bindersunder pressure at ambient temperatures into pellet form.

BACKGROUND OF THE INVENTION

The crystalline (mono-crystal or multi-crystal) silicon materials usedfor semiconductor as well as for photovoltaic devices manufacturing areproduced by crystal growth from melt. The feedstock for the siliconcrystal growth process is high purity silicon fabricated or produced byhigh temperature decomposition of silicon containing chloride (such astrichlorosilane or mono-silane).

The forms of the silicon feedstock that come directly from siliconmanufacturers are generally chunks and chips (from the breaking down oflarge silicon rods) and silicon beads (size ranges from several hundredmicrometers to millimeters). Such feedstock can be packed well into thecrucibles that are used for melting the silicon charges with a packingfactor of more than 50%.

The silicon beads are generally made by a fluidized bed process. Thisprocess also produces ultra-fine silicon powder (size ranges fromsub-micron to several hundred micrometers) as a byproduct in addition tothe useful silicon beads. The ultra-fine silicon powder is normallyreferred as Cyclone powder or filter powder depending on where it isdeposited; the range of sizes for the two powders is also different. Sofar there is no effective way to utilize this ultra-fine powder hencethe effective silicon conversion yield of the fluidized bed reactors islow.

Such ultra-fine silicon power has a purity as high as that used for thecrystal growth. However, because of its submicron size, it has a bulkdensity of about 0.25-1 gm/cc which is significantly low when comparedto the silicon solid density of 2.33 gm/cc. Because of the loosely boundnature of the powder the crucible cannot be loaded with more powder. Forexample, a 69×69×42 cm crucible can hold up to 300 Kg of solid silicon,while the powder can be charged only up to a maximum of 150 Kg.Conventional melt replenishment by continuous feeding of chips or beadsis also not possible with the above said powder because of the looselybound nature. Lack of proper way to utilize the ultra-fine siliconpowder renders such by-products of much less value. Furthermore, the lowbulk density presents storage problems due to requirements of largespace.

Such ultra-fine silicon powder can also be formed by homogenous thermaldecomposition of silicon containing gases (such as mono-silane). Thishomogenous decomposition is a much cheaper process comparing to theheterogeneous deposition process used in the Siemens and the fluidizedbed reactors. Therefore, a practical method of charging and feeding theultra-fine silicon powder would have a significant impact on the cost ofmanufacturing the silicon feedstock. This is especially important forphotovoltaic applications where cost reductions will make this renewableenergy source viable for terrestrial applications.

Another problem associated with ultra-fine powders is the very largesurface area; this results in an oxide coating on the powders. When suchpowders are heated to melt temperatures in ingot growth furnaces thispresents problems in melting, reactions with the hot zone anddegradation in performance of the silicon when used for solar cells.Therefore, ultra-fine powder additions to ingot growth charge have beenlimited to approximately five percent (5%).

The rapid growth of photovoltaic industry and current severe shortage ofsilicon feedstock has forced the manufacturers of crystalline silicon touse all sorts of silicon feedstock in all kinds of forms. This includesthe mixing of silicon powder, generally of larger than 100 micrometers,with other forms such as chunks of silicon. However, the use of theultra-fine silicon powder (sub-micron size) remains a challenge due toits very low bulk density. Moreover, the ultra-fine silicon can easilyflow with gases, which make it very hard to handle. Typically, the firstoperation in an ingot growth furnace after loading the charge isevacuation of the chamber; this can result in sucking the ultra-finepowders out of the crucible.

Compacting of silicon powder has been mentioned in several publications,as listed in the references. Both Möller's paper, Sintering of UltrafineSilicon Powder, and Takatori's paper, High Pressure Hot-Pressing ofSilicon Powders, were specific sintering, i.e., with elevatedtemperature to achieve full densification. Both papers are focused onthe grain boundaries formed after the high temperature sintering due totheir interests in the mechanical properties of the formed bulk siliconmaterials.

In Möller's paper, the starting silicon powder has sub micron particlesizes in the order of 0.02 to 0.1 μm. A pre-shaped compaction of siliconpowder, of little or no description, was made and subjected to hightemperature sintering without pressing. The densification andmicrostructure development (grain size, grain boundary transformation,etc.) were investigated under different sintering conditions forpurposes other than those of this invention.

In Takatori's paper, the compaction of silicon power was achieved byapplying pressure and elevated temperature simultaneously. As known tothe materials researchers, high temperature helps bulk diffusion as wellas the grain-boundary diffusion, and thus helps the densification of thesintered material. In fact, as reported in this paper, sintered densityof close to 100% of theoretical was achieved at temperature above 677°C. when combined with high pressing pressure (>1 GPa).

What is needed is a cost effective method and product for reclaimingultra fine silicon powder of high purity for uses requiring silicon of ahigh degree of purity.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a viable and practicalprocess and machinery to convert otherwise unusable ultra fine siliconpowder into a form factor, typically pressed pellets, that can beutilized in silicon melting processes. It is a further object to providea process and machinery that will maintain the purity of the silicon tonearly the same level as the starting powder, either in the pellet formdirectly or in the consumption of the pellet for its intended purpose.

It is another object of the invention to provide a system and facilityfor conducting a powder-to-pellet conversion on a commercially usefulproduction rate, such as high speed pellet pressing of up to 600 pelletsor more per minute, and processing of up to 50 kg or more of siliconpowder per hour.

It is an object of the compacting of ultra-fine silicon powder toenhance the packing factor of the silicon feed material in a cruciblethrough the use of compacted silicon pellets.

Another objective of compacting the ultra-fine silicon powder is toenhance the thermal conductivity of the powder so that the silicon feedstock can be easily heated and melted.

It is an object of the present invention to provide a process for theproduction of pellets having the structural integrity to withstandpackaging, handling and further processing by compressing silicon powderat an effective force, according to one embodiment, about approximately10,000 N (1.12 US Tons) to achieve adequate adhesion between siliconparticles.

One requirement for the resulting silicon pellets (compacts) is purity.The purity of the silicon in pellet form has to be maintained to nearlythe same level as the starting powder in order for the pellets to beuseful for making high purity silicon crystals. The silicon crystals aregenerally used for making semiconductor devices and solar cells. In someapplications, a small amount of binder may be tolerable. In otherapplication, small amounts of binder may be removed during consumptionof the pellet.

One aspect of this invention is the development of a process that addssignificant value to the silicon dust (ultra-fine powders) byproductfrom the Fluid Bed and other processes.

The universal method of treating fine powders is by addition of organicand inorganic binders to convert the powder to granules and pellets, asare practiced in ceramics industry, certain powder metallurgicalindustry, and pharmaceutical industry. These processes adddifficult-to-remove ingredients to the compact, and which in someprocesses are just contaminants to the material.

However, in one aspect of the process of this invention, silicon ispressed dry without addition of any outside material and at ambienttemperature. It thereby keeps the purity of the pressed pellet veryclose to that of the starting material. It is the combination of theability to convert silicon dust into compressed form by a dry no-bindertechnique that enables subsequent value-added use of the by-productsilicon powder, for example in the silicon melting processes ofphotovoltaic and electronic applications.

The only other application of dry pressing of powders is in themanufacture of nuclear fuel oxide pellets by the MOX process. Even inthis process small quantities of zinc stearate are utilized as anadditive to provide for initial agglomeration and pellet strength whilealso serving as a lubricant in the pressing operation. It is removed inthe subsequent high temperature sintering step.

In another aspect of the invention, a binder may be introduced into thepowder to facilitate the forming of the pellets. Some or all of thebinder may be removed during the pellet forming process, as by the heatof compression or the addition of a small amount supplemental heat.

Compacting fine powders at an elevated temperature (sintering) isanother general practice for obtaining densified components. Sinteringat high pressure (hot press) can achieve close to 100% density materialand the fine grains obtained from hot pressure provide the neededstructural strength. However, the application of the disclosed processhas minimal structural requirements and the grain boundary is notimportant for the resulting pellets. Additionally, in an elevatedtemperature environment, impurities from the die can diffuse into theformed silicon pellets, and thus contaminate the feed stock. During heatup to high temperatures the oxide layer on ultra-fine powders can leadto reactions leading to silicon carbide formation, thereby resulting incontamination and degradation in performance.

One process of this invention utilizes binder-less cold pressing ofsilicon powder without additives to form pellets that can be utilized insubsequent silicon processes. However, an organic binder may be used insome cases, and be all or partially evaporated during or after forming.The exact pellet shape and size, or uniformity of shape and size, is notof particular importance except as may be dictated by the availablepressing machinery. The structural integrity of the pellets needs onlybe sufficient to tolerate packaging and handling. There are no publishedreferences known to the Applicants that purport to utilize a process foreffective use of otherwise unusable silicon dust.

One aspect of the invention is that the material purity may bemaintained in the powder-to-pellet conversion operation, so long as theprocess is a binder-less dry method. In another aspect of the invention,any remaining binder may be removed in the consumption of the pellets,so that the resulting silicon product is of high purity not withstandingthe use of binder in the feed stock pellets.

The form factor of the pellet is important, because the pellets havesignificantly less surface area as compared to the ultra-fine powdersand, therefore, do not cause significant reactions during heat up. Thisminimizes contamination and degradation in performance and allows largerproportions (up to 100%) in the charge for ingot growth.

This invention is important because it provides a method to convert highpurity, but otherwise wasted, ultra fine silicon powder into a form thatis transportable in bulk, pure, and usable as feed material to siliconmelting processes, and in particular those processes requiring highpurity silicon.

In one aspect of the invention, the ultra fine silicon powder istransferred into a clean feed hopper attached to a pellet press machinesuch as a high quality Courtoy-type rotary indexing die and punchmachine. Controlled quantities of the powder are fed into the die by useof an appropriate powder feeder. Since the powder is ultra fine, aspecial powder feeder may be required. The powder is pressed by thepunch with a press force of several tons. No binder or additive isnecessary to the process, although for some applications an organicbinder may be used in the initial forming stage, and subsequently beevaporated during or after the pellet is formed.

The pressed pellet is ejected into a clean collection bin andtransferred into a lined shipping container. The pelleting machinery isequipped for automated and controlled operation. In addition, the entireprocess zone is located inside a controlled enclosure to maintainprocess and environment quality. The process facility also providescontrolled ingress and filtered egress for environmental safety.

Since no binder or additive is required in converting the ultra finesilicon powder to pressed pellet form, only nominal pellet strength,satisfactory for the purpose of compaction and transfer to secondaryoperations, is necessary. Some surface dusting and occasional breakageof pellets still provides an acceptable yield of the high-value endproduct, the dry, high purity silicon pellets.

In other and various aspects of the invention, there is no minimumparticle size for the powder. A small amount of binder may be added tothe powder to facilitate forming of the pellets. A small amount ofsupplemental heat may be applied during the pellet forming process,during which some or all of the binder may be evaporated. What binderremains may be partially or fully evaporated during heating of thepellets for consumption, so that the purity of the silicon product isaffected only slightly if at all.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is susceptible of many embodiments. As listed in Table 1,the results of one of several tests conducted by the Applicant, showthat an ultra fine silicon powder of median particle size of about 13micrometers, bulk density 0.56 g/cc (grams per cubic centimeter) and tapdensity about 0.68 g/cc, is converted into pellets of size 14 mm(millimeter) diameter×10 mm height, with a pellet weight of 2.3±0.05 g.The pelleting data for Table 1 was taken from pellets made with a cleanCourtoy R 53 rotary multi-station pellet press with compression forcecapacity of up to 14 tons. The tool set is made out of Tungsten Carbide.TABLE 1 Pellets 1 Pellets 2 Pellets 3 Pellets 4 Starting powder median13.5 μm 3.97 μm 4.97 μm 11.5 μm size Powder bulk density 0.56 0.58 0.650.34-0.58 (g/cc) Pellet diameter (mm) 14 18 18 12 Pellet height (mm) 9.5-10.0 5.3 5.3 4.1 Pellet density (g/cc) 1.49-1.58 1.41 1.41 1.72Pellet density (% of bulk 64-68% 60% 60% 74% Si) Compression force (N)10,000 10,000 10,000 10,000 Pellet purity NA NA NA GDMS, Close to powder

Table 1 illustrates silicon pellets made with different powder sizes andpellet sizes. The actual pellet size is not critical, and uniformity insize is not critical. In one embodiment, a precise quantity by weight ofthe silicon powder is fed into the die as a unit charge, andprogressively compressed by a matching punch to the required force toachieve the pre-calculated dimension that represents the desired finalpellet density based on the weight of the unit charge of powder fed intothe die. Alternatively, the process may be operated on the basis ofcompressing a unit charge that is sized as a pre-calculated volume ofpowder, and/or compacting the unit charge to a pre-calculated finalpressure or volume. The process criteria may be selected by calculationand/or by trial and testing of a useful range of the listed variables inTable 1 to achieve the desired pellet product.

To produce silicon pellets having the mechanical integrity to withstandpackaging, handling, storage and further processing, the silicon powdermust be exposed to an effective pressure to insure that there isadequate adhesion between the silicon power particles. By compressingthe powder, the silicon atoms of different particles are in sufficientlyclose proximity to permit bonding or attraction between the atoms and asa result, the particles. The compression packs the particles closertogether, eliminating voids and holding the particles, and the atoms ofwhich they are comprised, in close proximity. While this packing neednot be the highly ordered packing of crystalline silicon, higher degreesof order and of bonding produce more mechanically stable and purerpellets. According to one embodiment a force of about approximately10,000 N (1.12 US Tons) was used. Those skilled in the art will readilyappreciate that the forces in excess of 10 KN would likewise producepellets of sufficient structural integrity, as would weaker forces solong as the particles are compacted sufficiently to adhere withoutadditives.

For example, the starting power particle size may be expected to rangeup to 20 μm and more. The power density may range from less than 0.60g/cc to more than 0.75 g/cc. The compressive force, exact pelletgeometry and volume, and final pellet density may be a function of theavailable machinery, but testing suggests that a good pellet can beproduced from the range of powder specified, with 10,000-20,000 Newton'sin a volume range of about 0.5 to 2.6 cubic centimeters, weight range ofabout 1.0-3.0 grams, and a density range of about 50-75% of thetheoretical density of elemental silicon. Extrapolating our testresults, it is expected that an average pellet density of at least 50%will be necessary to survive bulk packaging and handling. Densities ashigh as 85% are expected to be attainable with smaller particle sizesand higher compression forces. Our test results demonstrated that thesmaller the size of the powder, the better integrity of a formed pellet,or the easier for compacting.

As another example, For example, the starting power medium particle sizemay be expected to range up to 20 μm and more. The power density mayrange from less than 0.60 g/cc to more than 0.75 g/cc. To the siliconpowder there may be added an organic binder. Organic binders such asAcetone, or Alcohal (methyl, ethyl or iso propyl) or isobutylmethacrylate mixed with ethylene dichloride and carbon tetra chloride,and others, may be used to facilitate forming.

The organic binders such as acetone or alcohol provide a good bindingforce between the silicon particles and easily get evaporated at roomtemperature without leaving any significant residues.

Other binders such as cellulose ethers (solid binder or added withalcohol solvent) need a high temperature bakeout to burn off the binder.The time for weight loss upon heating can be determined by differentialthermal analysis. For cellulose ethers the complete burn off occurs at400 degrees centigrade, cleanly and predictably with first orderkinetics. All binders added should be of electronic grade.

The compressive force, exact pellet geometry and volume, and finalpellet density may be a function of the available machinery, but testingsuggests that a good pellet can be produced from the range of powderspecified, with 10,000-20,000 Newton's in a volume range of about 0.5 to2.6 cubic centimeters, weight range of about 1.0-3.0 grams, and adensity range of about 50-75% of the theoretical density of elementalsilicon. Extrapolating our test results, it is expected that an averagepellet density of at least 50% will be necessary to survive bulkpackaging and handling. Densities as high as 85% are expected to beattainable with smaller particle sizes and higher compression forces.Our test results demonstrated that the smaller the size of the powder,the better integrity of a formed pellet, or the easier for compacting.

According to one embodiment of the present invention dry silicon powderis used. The presence of moisture might result in clumping, orimpurities that might have a deleterious effect on the purity of thefinal product, the integrity of the pellet, or the operation of the die.Those skilled in the art would, however, readily appreciate that the useof wet silicon powder would be within the scope of the invention.

The compressed pellets are ejected from the machine through the take-offsystem. The pellets provide a loose bulk material form of silicon formelting for high purity silicon requirements. The powder-to-pelletconversion is accomplished dry, with no added ingredient in the process.This is required to maintain the purity of the silicon for subsequentuse.

The term “pellet” is herein inclusive of any form factor and descriptiveterm that implies a compacted small volume of the raw material, thepellets produced in quantity in the nature of a loose granular bulkmaterial that facilitates easy handling methods and ready conformance tocontainer shapes.

The invention is further extended by the utilization of the pelletizeddry silicon in the making of high purity silicon ingots. The suitabilityof the high-purity dry-compacted pellets for melting into high puritysilicon ingots is effectively demonstrated by the method of oneembodiment of containing the pellets in a fused quartz crucible, bakingin vacuum at 1350° C., and then melting in an inductively heatedgraphite susceptor system as is well known in the art. The melt is takenup to 1600° C., and then cooled. The resulting ingot is very bright andshiny, with no inclusions in it. There may be a trace of residual oxidematerial as slag on top center of the melted ingot. Dry silicon pellets,pressed from silicon powder, were first melted to form an ingot of highpurity silicon on Jan. 28, 2002.

The basic steps of another embodiment method for making high puritysilicon pellets is as follows: providing a source of high purity siliconpowder, feeding the powder into a blender, operating the blender toremove agglomerates, discharging the powder into a hopper, feeding acontrolled amount by weight or volume of the powder into a die, drycompacting the powder with pressure, exclusive of any additives orwetting agents, and then discharging the dry pellet from the die. Themachinery may be configured to operate multiple lines of multiple dies,to meet high volume requirements.

The further steps of this embodiment of making high purity siliconingots from the dry silicon pellets is conventional except for the useof the dry silicon pellets and the resulting purity of the ingots:containing a suitable number of the silicon pellets in a fused quartzcrucible, baking the crucible with pellets in vacuum at about 1350degrees C., melting the pellets at up to about 1600 degrees Centigradein an inductively or resistively heated graphite susceptor system, andcooling the melt so as to produce an ingot.

In the disclosed invention, in distinction to the work of Möller andTakatori discussed previously, the grain boundaries of the pellets is ofless importance because the formed silicon shapes are to be used as feedstock for melting. Therefore, no sintering is necessary as the disclosedprocess can obtain reasonably aggregated silicon pellets. Hightemperature sintering may introduce impurity into the silicon pellets,which is highly prohibited in the application of the disclosed process.Also, the invention disclosed does not require elevated temperatures,although supplemental heat may be added in some cases. The achieveddensification is in the range of 60-75% of theoretical, which has beenshown to be dense enough for the designated applications.

Other and various embodiments will be evident to those skilled in theart, from the specification, abstract, figures and claims that follow.

1. A silicon pellet for making silicon ingots, consisting of a drycompacted volume of silicon powder, said silicon powder having a medianparticle size within a range of up to about 100 micrometers and a rangeof particle size of not greater than about 1000 micrometers.
 2. Thesilicon pellet according to claim 1, said powder being free ofintentional additives and binders.
 3. The silicon pellet for makingsilicon ingots according to claim 1, said silicon powder having a medianparticle size within a range of up to about 10 micrometers and a rangeof particle size of not greater than about 1000 micrometers.
 4. Thesilicon pellet for making silicon ingots according to claim 1, saidsilicon powder having a bulk density within a range of about 0.60 g/ccto about 0.75 g/cc and preferably about 0.68 g/cc.
 5. The silicon pelletfor making silicon ingots according to claim 1, said silicon powderhaving a bulk density of about 0.68 g/cc.
 6. The silicon pellet formaking silicon ingots according to claim 1, said pellet having a densityof about 50-85% of the theoretical density of elemental silicon, andhaving a weight within a range of about 1 gram to about 3 grams.
 7. Thesilicon pellet for making silicon ingots according to claim 1, saidpellet having a diameter in the range of 10 mm to 20 mm, and a height inthe range of 5 mm to 15 mm.
 8. The silicon pellet for making siliconingots according to claim 1, wherein said powder comprises substantiallypure silicon particles.
 9. The silicon pellet for making silicon ingotsaccording to claim 8, wherein said powder is compacted with a forcesufficient to cause adhesion between said particles.
 10. The siliconpellet for making silicon ingots according to claim 8, wherein saidpowder is compacted with a force of at least approximately 10,000Newton's.
 11. A silicon pellet for making silicon ingots, comprising adry compacted volume of silicon powder compacted by a force of at leastapproximately 10,000 Newton's, said powder having a median particle sizeof up to about 10 micrometers, said pellet having a density of aboutapproximately 60-75% of the theoretical density of elemental silicon anda weight of about approximately 2.3 grams, said pellet having a diameterof about approximately 14 mm, and a height of about approximately 10 mm.12. The silicon pellet of claim 11, said pellet being free of additivesand binders.
 13. A method for making a silicon pellet comprising:providing a source of high purity silicon powder having a medianparticle size within a range of up to about 100 micrometers and a rangeof particle size of not greater than about 1000 micrometers; feeding acontrolled amount of said powder from said source into a pellet die;compacting with pressure said controlled amount of said powder in saiddie thereby forming a dry pellet of high purity silicon; and dischargingsaid pellet from said die.
 14. The method for making a silicon pelletaccording to claim 13, said compacting being done at about ambienttemperature.
 15. The method for making a silicon pellet according toclaim 13, said powder being free of intentional additives and binders.16. The method for making a silicon pellet according to claim 13 whereinsaid step of dry compressing comprises applying a force greater than orequal to about approximately 10,000 Newton's to said powder.
 17. Asilicon pellet for the making of silicon ingots produced by a processcomprising the steps of: providing an agglomerate, additive and binderfree source of high purity silicon powder having a median particle sizewithin a range of up to about 100 micrometers and a range of particlesize of not greater than about 1000 micrometers; feeding a controlledamount of said powder from said source into a pellet die; dry compactingwith pressure said controlled amount of said powder in said die therebyforming a dry pellet of high purity silicon; and discharging said pelletfrom said die.
 18. The silicon pellet according to claim 17, said drycompacting being done at about ambient temperature.
 19. The siliconpellet according to claim 17 wherein said step of dry compactioncomprises applying a force of greater than or equal to 10,000 Newton'sto said powder.
 20. The method for making a silicon pellet of claim 17,further comprising: adding an organic binder to said silicon powder toform a mixture of binder and silicon, said controlled amount of saidpowder comprising a controlled amount of said mixture, said drycompacting with pressure comprising adding sufficient heat whereby saidbinder is evaporated.